There is much indirect evidence to suggest that the activation of E2F transcription factors, via alterations in the p16-cyclin D-Rb pathway, is a key event in the development of most human cancers. There is also accumulating evidence to suggest that E2F participates in a protective, apoptotic pathway that functions to eliminate cells that have lost normal cell cycle control. The involvement of E2F in both of these processes likely explains how the E2F1 gene can conform to the definitions of both an oncogene and a tumor suppressor gene. In preliminary studies, transgenic mice with E2F1 expression targeted to the skin by a keratin 5 (K5) promoter are shown to have both hyperplastic epidermis and a hair growth defect as the result of aberrant apoptosis. The K5E2F1 transgene cooperates with an activated ras transgene or p53 deficiency to induce spontaneous skin tumors. In sharp contrast, K5E2F1 transgenic mice are resistant to skin tumor development in two stage chemical carcinogenesis experiments. It is our hypothesis that increased E2F1 activity can contribute to tumorigenesis by promoting proliferation while it can also inhibit tumorigenesis through the induction of apoptosis. K5E2F1 transgenic mice provide a unique model for studying both the oncogenic and tumor suppressive activities of E2F1 in the same, well-characterize mouse skin system. The objectives of this proposal are to test the above hypothesis and to define the factors that determine whether increased E2F1 activity will positively or negatively regulate the oncogenic process. Specific Aim 1 is to define conditions and cooperating factors that influence how E2F1 modulates tumorigenesis by using K5 E2F1 transgenic mice in carcinogenesis and genetic crossing experiments. Specific Aim 2 is to examine growth regulation and gene expression in primary transgenic keratinocytes to address mechanistic questions on E2F1 regulation of cell proliferation, differentiation and apoptosis. Specific Aim 3 is to develop transgenic models that will separate E2F-induced proliferation from E2F-induced apoptosis to examine the effects of each independently on tumorigenesis. This will be accomplished by generating transgenic mice expressing E2F2, another E2F family member that induces proliferation but not apoptosis, and E2F1 (1-374), a transactivation domain mutant that induces apoptosis but not proliferation. These new transgenic models will also be useful in examining regulatory pathways and target genes uniquely regulatory pathways and target genes uniquely regulated by E2F1, E2F2 or through E2F-mediated transcriptional repression. A long-term goal of these studies will be to use these transgenic models, and the information obtained from them, to design and test cancer therapies that will block the oncogenic activity of E2F1 while enhancing its tumor suppressive function.